1. Technical Field
The invention relates to computer networks. More particularly, the invention relates to a computer network protocol.
2. Description of the Prior Art
TCP/IP is a well known information exchange protocol. See, for example, J. Postel, Internet Protocol, RFC791 (September 1981); J. Postel, Internet Control Message Protocol, RFC792 (September 1981); J. Postel, Transmission Control Protocol, RFC793 (September 1981); R. Braden, Requirements for Internet Hosts--Communications Layers, RFC1122 (October 1989); V. Jacobsen, R. Braden, TCP Extensions for Long-Delayed Paths, RFC1072 (October 1988); J. Postel, The TCP Maximum Segment Size and Related Topics, RFC879 (November 1983); and J. Mogul, S. Deering, Path MTU Discovery, RFC1191 (November 1990).
When a TCP connection sends a segment, the size of that segment is limited by the Maximum Segment Size (MSS) and the path Maximum Transmission Unit (MTU). The MSS used by the local TCP protocol stack is set during the establishment of each TCP connection. The MSS used by each station is determined independently using the following algorithm:
Each end station sends a desired MSS to the remote endpoint in the SYN packet. This desired MSS is the MTU of the interface being used, minus the IP and TCP frame overhead (typically 40 bytes). PA1 Each end station then derives an MSS value to be used, which is the minimum of the MSS received in the SYN packet and the MTU of the local interface, minus the IP and TCP frame overhead.
When two networks having different MTUs are connected, each resulting MSS used by each end station is the smaller of the two MSSs. In addition, if the MSS is larger than the empirically determined path MTU, only segments conforming to the path MTU are sent. This allows communications to occur without the need for IP fragmentation, but removes the efficiency and performance advantages provided by interfaces having larger MTUs.
It would be desirable to allow a host using the TCP transport protocol to choose the larger of two unequal MSS values received during the MSS negotiation when establishing a connection between two stations.